Near Infrared Emission from Monomodal and Bimodal PbS

Article pubs.acs.org/JPCC

Near Infrared Emission from Monomodal and Bimodal PbS Nanocrystal Superlattices Michela Corricelli,† Francesco Enrichi,§ Davide Altamura,∥ Liberato De Caro,∥ Cinzia Giannini,∥ Andrea Falqui,⊥ Angela Agostiano,†,‡ M. Lucia Curri,† and Marinella Striccoli*,† †

Istituto per i Processi Chimico Fisici (IPCF-CNR) Bari, c/o Department of Chemistry, Via Orabona 4, I-70126 Bari, Italy Department of Chemistry, University of Bari, Via Orabona 4, I-70126 Bari, Italy § Coordinamento Interuniversitario Veneto per le Nanotecnologie (CIVEN) & Laboratorio Nanofab, via delle Industrie 5, I-30175 − Marghera (VE), Italy ∥ Istituto di Cristallografia (IC-CNR), via Amendola 122/O, I-70126 Bari, Italy ⊥ Fondazione Istituto Italiano di Tecnologia (IIT), via Morego 30, I-16163 Genova, Italy ‡

S Supporting Information *

ABSTRACT: PbS colloidal nanocrystal (NC) assemblies with monomodal and bimodal size distribution have been fabricated by slow evaporation of solvent on silicon substrates. The interparticle distances of the assembled structures have been carefully defined, both in the plane and in the z direction, perpendicular to the substrate, thanks to the combination of small and wideangle X-ray diffraction and TEM measurements. The spectroscopic characteristics of PbS NCs, both in solution and organized in a superlattice, have been investigated by steady-state and time-resolved photoluminescence measurements. The optical results reveal the occurrence of a Förster resonant energy transfer (FRET) mechanism between closed-packed neighboring PbS NCs. The occurrence of FRET is dependent on NC assembly geometry, and thus on their interparticle distance, suggesting that only when NCs are close enough, as in the close-packed arrangement of the monomodal assembly, the energy transfer can be promoted. In bimodal assemblies, the energy transfer between large and small NCs is negligible, due to the low spectral overlap between the emission and absorption bands of the different sized nanoparticles and to the large interparticle distance. Moreover, recombination lifetimes on the microsecond time scale have been observed and explained in terms of dielectric screening effect, in agreement with previous findings on lead chalcogenide NC optical properties.

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INTRODUCTION In the last years, growing attention has been addressed to nanosized materials for their unique physical and chemical sizeand shape-dependent properties. Among the original characteristics of nanocrystalline semiconductor materials, their optical properties have a great potential both in fundamental studies and in practical applications. When nanocrystal (NC) size is smaller than the Bohr radius, electronic quantum confinement effects arise and electronic transitions become quasi-discrete, showing a progressive increase of band gap at decreasing NC size, resulting in size-dependent absorption and emission features in the optical spectra. Namely, NC photoluminescence (PL) bandwidth is quite narrow and the emission spectral © 2012 American Chemical Society

position can be easily modulated by properly tuning the NC size.1 In this perspective, a fundamental role is played by the synthetic methods which have to ensure a precise control on the final NC size as well as a high crystalline quality and an accurate tailoring of surface chemistry properties of the NCs. Indeed, the development of improved synthetic protocols has fostered their application in many fields as biological targeting,3,4 LED devices,5,6 as well as photodetectors and Received: January 16, 2012 Revised: February 17, 2012 Published: February 21, 2012 6143

dx.doi.org/10.1021/jp300509f | J. Phys. Chem. C 2012, 116, 6143−6152

The Journal of Physical Chemistry C

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photovoltaics.7,8 However, a high percentage of defects can sensibly increase the probability of alternative nonradiative deexcitation pathways, thus decreasing the fluorescence quantum efficiency of the NCs.2 Among the different materials, IV−VI semiconductor NCs, and in particular lead chalcogenide NCs, are very appealing, due to their emission in the near-infrared (NIR) region and the strong confinement of charge carriers, ascribable to their very large Bohr radius (aB = 18 nm for PbS NCs).9 For this reason, large quantization energies are expected, which allow tuning the transition energies of the NCs across a large portion of the NIR spectrum. Indeed, lead chalcogenide NCs have been investigated both in fundamental studies as well as in a series of applications requiring strong confinement, including investigation of optical properties of core−shell PbSe@PbS NCs10,11 and of amplified spontaneous emission from closely packed PbSe NCs,12 potentially useful for tunable infrared lasers. Moreover, the synthesis of water-soluble PbSe NCs for biological applications13 and the employment of PbS14 and PbSe NCs15 as infrared active element of LEDs has been reported. In the past few years, semiconductor NCs have been progressively considered as building-blocks to flexibly use for the fabrication of large and ordered artificial solids.16,17 Such nanocrystalline elements, once organized in assemblies, are able to develop new original collective properties.18 Indeed, NCs closepacked in a solid permit a much more effective coupling between neighboring nano-objects, than that occurring in solution.19,20 Such coupling can result in Förster resonant energy transfer (FRET), i.e., a radiationless nanoscale mechanism by which energy can be transferred from a donor to an acceptor, by means of dipole−dipole interactions.21 However, while the FRET mechanism for organic molecules has been widely investigated, the spectroscopic behavior of such energy transfer mechanism still needs to be fully elucidated for NCs. Luminescent semiconductor NCs are versatile systems, displaying tunable emission wavelength, very narrow PL peaks and extremely high photostability, conversely to organic dyes, characterized by broad band absorption spectra. In addition, NCs of few nanometers size, assembled onto substrates, are characterized by interparticle distances in the range where FRET typically occurs.22 Indeed, NCs possess an intrinsic, although controlled and very narrow, size polydispersity, which results in a broadening of the absorption and emission line shape and in interaction and possible energy transfer between nano-objects slightly different in size. In this perspective, achieving a precise arrangement of NCs into defined 2D and 3D geometries, together with an accurate control on interparticle distance, is of primary importance. Such a control can be performed by selecting an appropriate assembly technique and by cleverly playing with relevant parameters, such as NC size, concentration, capping agent, dispersing solvent, and deposition temperature, which can influence the NC organization in the final structure. Several examples of organization of luminescent NCs in superstructures, fabricated by using different techniques, such as dropcasting,23,24 spin-coating,25 and electrostatic layer-by-layer deposition,26,27 have been reported. In the present work, we take advantage of an original synthetic procedure28,29 which allows us to obtain, by just varying the synthetic parameters, a very narrow monomodal size-distribution (MSD) or, alternatively, a bimodal size distribution (BSD) of PbS NCs, characterized by two well-defined and highly monodisperse populations.

The prepared NCs have been self-assembled onto a suitable substrate, by using a solvent evaporation procedure, and superlattices with a defined geometry dependent on the PbS NC size and size ratio in the bimodal population have been obtained, as confirmed by the TEM micrographs and small angle XRD investigations. For the first time to our knowledge, the evaluation of the interparticle interaction energy has been carried out on a system with fully established geometrical arrangement. Indeed, the interparticle distances in the investigated systems, have been well-defined also perpendicular to the substrate, in the z direction, and not just in the xy plane. An in-depth investigation has been carried out on the UV−vis− NIR absorption and NIR-PL emission properties of PbS NCs, both monomodal and bimodal in nature. Such characteristics have been evaluated against the corresponding properties of NCs in solution, also as a function of the solvent, as well as the NC size and size ratio in the bimodal samples. The comparison of the energy position of the emission spectra, moving from the NC solution to the deposited solid, has provided insights on possible FRET phenomena between neighboring NCs. The occurrence of FRET has been demonstrated to be dependent both on the energy level overlap and on the geometry of NC assemblies, i.e., NC size and interparticle distance. Indeed, FRET has been observed for PbS NCs assembled in a monomodal superlattice, between the slightly larger and smaller NCs, belonging to the same population. On the other hand, no FRET was experimentally observed in PbS NC bimodal superlattice, for both geometric and energetic reasons.

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EXPERIMENTAL SECTION MATERIALS. Lead(II) oxide (PbO, powder 99.99%), hexamethyldisilathiane (HMDS, synthesis grade), trioctylphosphine (TOP, 90% technical grade), toluene (laboratory reagent, ≥99.3%), tetrachloroethylene (TCE, >99%), and chloroform (≥99.8%) were purchased by Sigma-Aldrich and used as received. 1-Octadecene (ODE, 90% technical grade) and oleic acid (OLEA, technical grade 90%), also purchased by SigmaAldrich, were distilled before use. PbS Nanocrystal Synthesys. The synthesis of PbS NCs was carried out as previously reported.28,29 In a typical synthesis, 0.6 mmol of PbO, 2.0 mL of TOP, and 1.3 mol of OLEA were added to 20 mL of ODE, a noncoordinating solvent, and stirred under vacuum at 120 °C: at this stage the formation of lead-oleate precursors occurred. Subsequently, a 20 mM solution of the sulfur precursor, hexamethyldisilathiane (HMDS) in ODE, was swiftly injected, corresponding to Pb/S molar ratio equal to 24:1 and 50:1, followed by cooling to 80 °C in order to stop the nucleation and let the freshly formed nuclei grow. The reaction was stopped at different times, according to the final desired NC size. After the synthesis, the obtained NCs were purified by using a nonsolvent precipitation procedure, carried out by adding to the reaction product a large amount of a short alkyl chain alcohol (ethanol). In order to completely remove the excess of OLEA and other impurities formed during the reaction, up to 3 centrifugation steps were required, finally obtaining a powder. A drying step is necessary to remove the residual alcohol molecules, which may interfere in the spectroscopic investigation, since their corresponding optical absorption bands may overlap with the characteristic signal of the PbS NCs in the spectral region between 1100 and 1800 nm. The obtained precipitate was then dispersed in different organic solvents such as toluene or TCE. In particular, TCE is particularly effective 6144



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10 Hz and a pulse duration of 6 ns. The system is coupled to an Optical Parametric Oscillator which is able to produce photons in the whole range 210−2300 nm. All the experiments have been performed at 500 nm pulsed excitation. The decay profiles were fitted with the least-squares method, by using a monoexponential or a biexponential equation, with a temporal constant τ or τ1 and τ2, respectively. For biexponential fitting, the weighted coefficients B1 and B2 were also obtained, and an average value of lifetime τa was calculated. In all cases, χ2 values close to 1 confirmed the high quality of the fitting procedure. X-ray Diffraction Analysis. Small angle XRD measurements were performed in coupled detector/sample scan mode (2θ/θ scans) on a D8 Discover diffractometer by Bruker, equipped with a Göbel mirror, using Cu Kα radiation. Wide angle XRD measurements were performed at fixed incidence angle (2θ scans) by using a diffractometer equipped with a 12 KW Rigaku 200H rotating anode X-ray source using Cu Kα radiation, monochromatized by a Ge single crystal (111 reflection), and a INEL curved position sensitive detector (CPS 120). Transmission Electron Microscopy Investigation. Transmission electron microscopy (TEM) analysis was performed by a Jeol Jem-1011 microscope, equipped with a high-contrast objective lens, a W electron source and working at an accelerating voltage of 100 kV. Under these conditions, the ultimate point resolution of the microscope was equal to 0.34 nm. The TEM images were acquired by a Gatan SC-1000 Orius Camera, equipped with a fiber-optical coupled 11 Mp CCD. TEM measurements were carried out both for investigating size and size dispersion of PbS NCs and for studying the characteristics of the superlattice. In the former case, 200−300 nanoparticles for each specimen were analyzed. For the latter, structures formed as result of the procedure described hereafter were investigated.

for redispersing NCs and does not has optical absorption features in the same spectral range of the PbS NCs. A precursor molar ratio Pb/S = 24:1 and a reaction time of 15 min led to a MSD of PbS NCs with diameters of 2.6 ± 0.5 nm, as measured from TEM images. Precursor molar ratio Pb/ S = 50:1 and reaction times equal to 8 min led to a BSD of PbS NCs with diameters equal to 1.6 ± 0.2 nm for the small and 3.9 ± 0.4 nm for the large nanoparticles, respectively. SUPERLATTICE FABRICATION. A proper amount of PbS NC solution, with a suitable concentration, was drop-cast onto both amorphous carbon coated Cu TEM grids (for TEM characterization) and silicon substrates (for XRD and PL characterization), allowing the solvent to evaporate, in open-air conditions, at room temperature and keeping the substrate horizontal. [The NC concentration in solution was calculated starting from the precursor molar content, the single PbS NC crystallographic structure (cubic rock salt, as confirmed to be for the prepared PbS NCs from the X-ray diffraction experiments), and the NC size. As the sulfur precursor was added in defect with respect to the lead one, the reaction yield was considered to completion for the former reactant, thus resulting in the assumption that PbS NC molar concentration was equal to the starting sulfur molar content. The Pb/S pair number for any particle size was thus calculated, and consequently, the molar weight of a single PbS NC was derived that allowed us to define the PbS NC concentration.] Both MSD and BSD solution samples were divided in two aliquots and, after purification, each redispersed in 2.5 mL of two different solvents, namely toluene and TCE. Both MSD and BSD PbS NC stock solutions were further diluted 1:90 and 1:30, in order to obtain two solutions, at low and high concentration, respectively. In particular, MSD PbS NC sample solutions 7.5 × 10−7 and 2.5 × 10−6 M were investigated. On the other hand, the concentration estimation for the BSD PbS NC sample, cannot be easily made, as the exact amount of the smaller and larger NCs cannot be calculated. A total of 150 μL of the obtained solutions were successively drop-cast onto silicon substrates, for both MSD and BSD samples, thus obtaining films from the lower concentration and the higher concentration solutions, defined as filmL and filmH, respectively. For comparison, the samples at lower concentration were also employed for solution measurements. UV−Visible−NIR Absorption Spectroscopic Investigation. The absorption spectra of PbS NCs were recorded from samples purified, dried and redissolved in TCE, by means of a Cary Varian 5000 UV−vis−NIR spectrophotometer. The sample purification and drying steps were needed in order to remove the interfering absorption from both unreacted molecules and the solvent used for the purification (ethanol). NIR Steady-State and Time-Resolved Photoluminescence Spectroscopic Investigation. The photoluminescence (PL) properties were investigated for both MSD and BSD PbS NCs. PL measurements were performed with a Horiba Jobin Yvon Fluorolog-3 spectrofluorimeter. The PL emission spectra were obtained by using a 450 W Xe lamp as excitation source, coupled to a double grating Czerny-Turner monochromator for wavelength selection. The detection system was constituted by a iHR300 single grating monochromator coupled to a R5509−73 Hamamatsu photomultiplier tube, able to cover the whole 300−1700 nm spectral range. Time-resolved analyses were performed in multichannel scaling modality (MCS) by using a tunable pulsed Nd:YAG laser system as excitation source, characterized by a repetition rate of

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RESULTS AND DISCUSSION It is well-known that semiconductor materials, in nanosized regime, show interesting size-dependent absorption and emission properties. The spectral response of NC superlattices may go beyond the mere properties of the individual NC component, resulting in collective properties deriving from the NC organization in superstructure.30 Therefore, spectroscopic investigations on MSD and BSD PbS NCs have been carried out both in solution, to get information on the NC component, and on NCs organized in thin films, in order to reveal possible occurrence of collective phenomena in the assemblies. In Figure 1a, the absorption spectrum of the monomodal size-distribution (MSD) PbS NCs in TCE solution shows a well-resolved exciton transition signal, localized at 1182 nm, with a full-width at half-maximum (fwhm) of 92 meV, which reflects a narrow size dispersion of the NCs, experimentally measured from TEM micrographs in the order of 6%. The corresponding photoluminescence spectrum is characterized by the presence of a single narrow emission peak, Stokes shifted of 31 nm with respect to the corresponding absorption band (Figure 1a). Such an emission peak can be attributed to bandedge recombination, whereas no signal can be ascribed to defect states. In Figure 1b the comparison between the excited state decays of MSD PbS NCs in toluene and in TCE solution, is reported. Lifetimes of the carriers in the order of microsecond, in particular equal to 1.20 ± 0.01 and 1.26 ± 0.02 μs, have been measured for MSD PbS NCs in toluene and in TCE solutions, 6145



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experimental results. Finally, the similar trends observed in the excited state decay of TCE and toluene PbS NC solution, can be reasonably explained by recalling the very close values of their dielectric constants. The UV−vis−NIR absorption and PL spectra of the BSD PbS NCs in toluene are reported in Figure 2a. The absorbance

Figure 1. (a) Comparison between the UV−vis−NIR absorption (black line) and PL emission spectra (blue line) of MSD PbS NCs in TCE solution. The excitation wavelength is 500 nm. (b) Timeresolved PL decays recorded at 1180 nm on the 2.6 ± 0.5 nm MSD PbS NCs in toluene (red) and TCE (green), respectively. The square symbols refer to the excited state decay, whereas the solid line is the corresponding fitting.

respectively. This indicates that, for the present sample, the decay is negligibly affected by the nature of the solvent. The lifetime values, in the microsecond range for both samples, confirm the very long excited-state lifetimes of PbS NCs, in good agreement with values reported in literature for similar lead chalcogenide systems, from both experimental measurements and theoretical calculations.31,32 Such notably long excited-state lifetimes are ascribed to a significant dielectric screening of the radiating field inside the NCs, which is very typical for materials with high dielectric constants, such as PbSe and PbS.33 Indeed, the theoretical dipole lifetimes of PbSe NCs in chloroform has been calculated to be of the order of ∼25 ns.33 Such a value, however, is still quite different from the τ values which can be experimentally recorded for PbSe NCs. This discrepancy has been justified by considering that screening of the radiating field inside a spherical NC weakens the internal field and ultimately increases the radiative lifetime by a factor equal to [3ε1/(ε2 + 2ε1)]−2, where ε1 and ε2 are the optical dielectric constants of the host matrix (solvent) and the NC material, respectively.33 Approximating the dielectric constant to the square of the refractive index and considering chloroform as NC host, a radiative lifetime value of ∼0.4 μs can be found for PbSe NCs. NC dispersion in a solvent different from the CHCl3 further influences τ, due to the different value of the solvent’s refractive index. In our study, PbS NCs are dissolved in toluene or in TCE solutions, having dielectric constants of 1.505 and 1.496, respectively, versus 1.446 of CHCl3. Therefore, the theoretical dipole τ is expected to increase by a factor of ∼30,34 with respect to the calculate one without screening effect. Indeed such a value is in agreement with our

Figure 2. (a) UV−vis−NIR absorption (black line) and PL emission spectra (blue line) of 1.6 ± 0.2 nm/3.9 ± 0.4 nm BSD PbS NCs in TCE solution. The excitation wavelength is 500 nm. (b) Timeresolved PL spectra of BSD PbS NC sample in toluene, at 810 nm (red), at 1040 nm (green), and at 1480 nm (blue). Scattered squares refer to the excited state decay, whereas the solid line is the corresponding fitting.

spectrum displays two absorption peaks, at 620 and 1465 nm, respectively, which can be clearly attributed to the coexistence of the two distinct PbS NC populations, having different size, namely 1.6 ± 0.2 and 3.9 ± 0.4 nm, respectively. On the other hand, the corresponding PL spectrum shows three distinct peaks at about 820, 1010, and 1480 nm. The analysis of the emission features, and their comparison with the absorption spectrum, suggest that the peaks at 820 and 1480 nm are due to the band gap excitonic recombination of the smaller and larger PbS NCs, with a Stoke’s shift from the absorption transitions of 200 and 15 nm, respectively. The rather significant difference in the Stoke’s shift values for the two PbS NC populations is consistent with the reported linear NC size dependence of the Stoke’s shift.35 The band at 1010 nm could be ascribed to shallow impurities in the smaller PbS NCs, which originate from surface defect states and are positioned at energy values between E1Se and E1Sh. The energy level of such a band, lower than the energy of the direct band gap transition of the small NCs, along with its broader line shape, is consistent with such an attribution. In fact, such a feature might be hardly ascribed to the emission of a third PbS NC population, since neither 6146



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Figure 3. TEM images at different magnifications of the assemblies obtained by drop cast from a toluene solution on a copper grid of MSD PbS NCs (a and c), schematized in the inset of (c). (b and d) BSD PbS NCs, schematized in the inset of (d).

by considering the organic soft shell onto the inorganic crystalline core (dNC):37 dNC + twice the length of OLEA (∼1.8 nm), in presence of OLEA/TOP as capping agents, corresponding to 6.2 nm, in agreement with the measured value. In Figure 3b,d the TEM images of a BSD PbS NC sample are reported. The 2D arrangement of the large NCs clearly presents a hexagonal symmetry; on the other hand the small NCs are not directly visible. As it will be explained in the following, each large NC is surrounded by twelve smaller ones, defining an AB6 stoichiometry of the assembly. The measured in-plane center-to-center distance between larger PbS NCs (which corresponds to the in plane lattice parameter) is 8.9 ± 0.3 nm. The NC organization in the direction perpendicular to substrate surface (out-plane order) has been investigated by small-angle X-ray diffraction, for both MSD and BSD samples (Figure 4, panels a and b, respectively). The variation of electron density approximately perpendicular to the substrate plane has been probed through coupled detector/sample scans (2θ/θ). The presence of equally spaced diffraction peaks (00l) in Figures 4, panels a and b, indicates that NCs are ordered along the out of plane direction in both MSD and BSD samples, with a mean periodicity d given by d = 1/ΔQ, where Q = 2 sin θ/λ is the length of the scattering vector, λ is the X-ray wavelength, and ΔQ is the distance between the Q values of two consecutive intensity maxima. In order to make peaks more clearly visible in Figures 4, panels a and b, a constant background has been subtracted, and the resulting patterns have been reported together with the original ones. In the case of the MSD sample (Figure 4a) four equally spaced peaks can be observed, thus demonstrating a well-defined out of plane stacking of similar layers spaced by cMSD = 5.62 ± 0.15 nm, as suggested by the monotonic decrease of the scattered intensity, with no particular modulation features. Conversely, for the BSD sample (Figure 4b), a more complicated intensity modulation can be recognized in the diffraction pattern. From the average spacing between consecutive peaks, a mean periodicity of

UV−vis−NIR absorption spectrum nor TEM investigations provide any evidence of such an additional NC population. Time-resolved PL measurements have been carried out on toluene BSD based PbS NC solution, and the recombination decays have been recorded in correspondence of the three different emission peak maxima (Figure 2b), obtaining τa values equal to 1.32 ± 0.04, 1.77 ± 0.21, and 3.40 ± 0.27 μs, respectively. The excited state lifetime corresponding to the decay recorded for the peak at about 1010 nm has been found to be slightly longer than that recorded at 820 nm, corresponding to the band gap recombination peak of the smaller PbS NCs. Therefore, the decay recorded at 1010 nm can be reasonably ascribed to a defect recombination. However, a significantly longer τ would have been expected for such a lifetime, compared to the corresponding value for the band-edge recombination. Such a discrepancy could be probably explained in terms of the dielectric screening contribution. Remarkably, the lifetime corresponding to the peak at 1480 nm and ascribed to the band-edge recombination of the larger PbS NC confirms the size-dependent trend of lifetime for PbS NCs, as predicted by theoretical calculations.36 Both MSD and BSD NC solutions, at two different concentrations, have been drop-cast onto silicon substrates and the solvent left slowly evaporate. The concentration values have been selected on the basis of a previous study,28 which demonstrated that the higher is the PbS NC concentration, the larger is the extent of the obtained ordered assembly. Under such conditions, NCs effectively organize in superlattices, as the thermodynamic contributions, deriving from the ordering, seem to compensate the entropic loss associated to the NC transition from solution to solid film. When PbS NCs, with a MSD of 2.6 ± 0.5 nm in toluene solution (2.5 × 10−6 M), have been deposited on a TEM grid, hexagonal close-packed films have been obtained, as shown in Figure 3a,c. The center-to-center interparticle distance, as derived from the TEM micrograph, is 6.7 ± 1.3 nm. The effective particle diameter can be calculated 6147



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Figure 4. Small-angle XRD patterns of MSD (a) and BSD (b) samples, as a function of the scattering angle (2θ, bottom axis) or the scattering vector length (Q, top axis); the identified peak positions are indicated by the (00l) series. Wide-angle XRD patterns of MSD and BSD samples, and expected peak positions for the cubic rock-salt PbS crystalline phase (c). Scheme of the proposed PbS NC arrangement in the BSD superlattice (d).

cBSD = 6.4 ± 0.1 nm along the normal to sample surface has been derived. A detailed combined XRD/TEM analysis, reported in ref 38, allowed to derive the diameters of the small NCs as well, which resulted equal to 1.6 ± 0.2 nm. Moreover, the NCs in the BSD sample have been demonstrated to be organized in a 3D superlattice, whose unit cell is expected to be as sketched in Figure 4d. Based on the results reported in ref 39, such unit cell has been defined as the most probable, where the mutual possible (center-to-center) distance between neighboring small NCs has been calculated to be 2.6 nm, although a larger mutual distance (3.0 nm) could locally occur (not for particles labeled 3 and 4). Such two different values are due to the fact that the vertical coordinate (perpendicularly to the substrate plane) of each small NC position in the unit cell could be in principle at (c/2) ± 0.74 nm (compare for example Figure S1a and S1b), as illustrated in ref 39. The most probable edge-to-edge distance between small NCs is therefore 1.1 nm, and can locally reach 1.5 nm (not between particles 3 and 4). On the other hand, the (center-to-center) distance between a large and a neighboring small NC would be in any case 5.2 nm and thus their edge-to-edge distance 2.5 nm. In Figure 4c are reported the wide-angle XRD experimental patterns of the two samples, which are shown to match the expected pattern for a cubic rock-salt PbS powder sample with a 5.93 Å lattice constant. As the NC concentration has been demonstrated to affect the extent of the lattice order, it is reasonable to think that the film prepared from the solution with the higher concentration, filmH, will be characterized by an overall higher order with respect to the film prepared form the solution with the lower

concentration, filmL. In other words, the filmH hosts higher extent of self-organized PbS NC domains than filmL. In addition, the XRD data confirm that the kind of substrate (silicon or TEM grid) does not affect the lattice formation and structure,28 thus allowing to infer that the structures observed by TEM can be directly related to the XRD structural and spectroscopic investigation. The influence of concentration and dispersing solvent, which have been found relevant in the superlattice formation, has been then evaluated based on the optical response of the related solid architecture. In Figure 5a the comparison between the PL emission spectra of the MSD PbS NC filmL and filmH is reported. A shift of PL emission of the two films toward wavelength higher than that of the PL peak of PbS NC in TCE solution can be noticed (Table 1). In particular, such a red-shift is more pronounced in filmL than in filmH. The different emission peak position in NC solution and films can be attributed to a different electronic arrangement due to the distinct chemical surrounding experienced by the NCs in solution and organized in solid films, respectively. Indeed, when the PbS NCs are in solution, they are almost free to move and only limited mutual interactions can be experienced. Conversely, for NCs organized in a film, interparticle distances are shorter than in solution, thus promoting more effective interparticle interactions. The additional red-shift observed moving from filmL to filmH can be probably due to a different extent of ordered domains in the two types of films, since in filmL a coexistence of truly close-packed zones and disordered areas, where NCs are irregularly distributed, can be assumed. 6148



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Figure 5. (a) Emission spectra of MSD NC based filmL (blue line) and filmH (red line) (excitation wavelength 500 nm). (b) PL emission spectra of BSD PbS NCs filmL (blue line) and the filmH (red line), drop-cast on silicon substrates (excitation wavelength 500 nm).

Figure 6. (a) Time-resolved PL spectra of TCE solution (red) and filmH (green) for MSD PbS NCs, recorded at 1180 nm. (b) Timeresolved PL spectra of toluene solution (red) and filmH (green) for the BSD sample, recorded at 1480 nm. The scattered squares refer to the excited state decay, while the solid line is the corresponding fitting.

Table 1. Wavelength Values Corresponding to the Principal Emission Peaks, for the MSD PbS NC Sample in Solution and Organized in filmL and filmH, Respectively (Excitation Wavelength 500 nm)

essentially from the solvent. Conversely, in the case of the solid film, the NC host medium is significantly different, as each NC mainly perceives the contribution from the surrounding NCs, rather than from the solvent. Approximating the dielectric constant of the medium to the square of the refraction index, the TCE and PbS dielectric constants are found to have considerably different values, namely 1.505 and 17, respectively, which can significantly affect the measured lifetimes. A further contribution to the difference in the observed lifetimes could also come from the concomitant FRET from smaller to larger PbS NCs, within the same NC population, in agreement with the steady-state PL results. In this case, the FRET phenomenon occurrence can be promoted by a good overlap of the NC energy levels (belonging to the same population) as well as by a favorable geometrical interparticle distance (as found out from TEM measurements), as reported in Figure 7a. This occurrence is further confirmed by the concentration-dependence of the excited state lifetime value (Figure S2a). Indeed, it has been reported that FRET results more efficient for films with higher NC concentrations.40 Also in the case of the BSD PbS NC sample, the comparison of the PL decay for samples in solution and as solid film from the concentrate solution (filmH), has been considered (Figure 6b). The decay profiles have been recorded at 1480 nm, with calculated lifetimes of 3.40 ± 0.27 μs for the solution and 1.54 ± 0.20 μs for the filmH. Such a trend looks quite similar to that observed for the MSD samples, which showed only a more pronounced lifetime decrease. However, no red-shift of PL peak has been observed either passing from solution to film or increasing the NC concentration in the film. In addition, the

λMSD (nm) solution filmL filmH

1182 1194 1242

Analogous measurements have been performed on BSD samples, and in Figure 5b the steady-state PL spectra of filmL and filmH are shown. In this case, moving from the solution to the solid films, the peaks at 820 and 1480 nm do not change significantly their positions. Conversely, moving from filmL to filmH, the PL signal at about 1010 nm, although retaining its broad shape, shifts to larger wavelengths. In addition, a change in the relative intensity with respect to the peak at 820 nm has been detected. Successively, a comparison between the excited state decay of MSD PbS NCs in TCE solution and organized in filmH has been carried out (Figure 6a). Interestingly, moving from the MSD PbS NC in solution to the filmH, a significant reduction in the lifetime has been observed. The measured lifetimes pass from 1.26 ± 0.02 μs for the MSD PbS NC in solution to 0.41 ± 0.02 μs for NCs in the filmH. The different lifetimes of solution and filmH, together with the increase in Stoke’s shift of steady state PL, can be explained on the base of two main factors, namely the different dielectric surrounding the NCs and the FRET. In fact, in the case of the MSD PbS NC in TCE solution, the dielectric contribution from the surrounding comes 6149



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could be safely ruled out, for both geometric and energetic considerations (Figure 7b). Particularly, the large interparticle distance (8.9 ± 0.3 nm) between the larger PbS NCs, belonging to the 3.9 ± 0.4 nm population, strongly reduces the probabilities of FRET between them. In addition, a FRET between the smaller PbS NCs, belonging to the 1.6 ± 0.2 nm population, has a low probability, due to their intrinsic very narrow size-distribution. Finally, since no overlap between the emission spectrum of the 1.6 ± 0.2 nm PbS NCs and the absorption spectrum of the 3.9 ± 0.4 nm PbS NCs can be seen, FRET between these NCs cannot be easily foreseen. Finally, Figure 8a reports a comparison between the lifetime decays of the filmL for the MSD PbS NCs, deposited from two

Figure 7. Sketch representing the energy transfer in the two different solid films, constituted by a MSD (a) and BSD PbS NCs (b). The NC geometry and the proposed energy transfer mechanism are reported. The yellow lines evidence the interparticle distances, while the purple arrows indicate the interparticle energy transfer.

NC concentration in the film seems not to affect the excited state lifetime (Figure S2b). In such a case, a FRET mechanism cannot be envisioned. The most probable explanation for the excited state lifetime decrease, passing from solution to film, can be assumed coming from dielectric screening effects. NC energy transfers have been previously investigated in assemblies of NCs homogeneous in size, as a function of size and their size distribution. For films formed of a mixture of NCs with different sizes, the occurrence of energy transfer phenomena has been inferred from the quenching of the PL emission of the smaller NCs and the concomitant increase in the PL emission intensity of the larger NCs. For monodisperse NC samples, possessing an intrinsic size-distribution, an energy transfer has been observed as a PL emission peak red-shift, with a simultaneous narrowing of the emission line shape, compatible with a unidirectional energy flow from smaller to larger NCs, and, at the same time, with a decrease of the PL excited state lifetime compared to NCs in solution.19,41 Being FRET a radiationless dipole−dipole energy transfer, it is triggered when a donor is brought in close proximity to an acceptor. The effectiveness of such an energy transfer depends on different factors, as the extent of the spectral overlap between the donor PL and the acceptor absorption, as well as on the interparticle distance (to the sixth power).42 Förster radii have been found on the order of 5−8 nm for close-packed PbS NCs with a size in the range 2−2.5 nm.23 In the case of FRET mechanism, both PL peak red-shift and shorter lifetime for excited state decay can be associated to an energy transfer between slightly smaller and larger NCs in the same size distribution, emitting on the blue and red sides of the assembly emission spectrum, respectively. The energy transfer can thus take place thanks to the overlap of the emitting and the absorbing states of the NCs assembly, made possible by the size distribution inhomogeneous broadening and the presence of a Stoke’s shift.19,43 The occurrence of FRET phenomena has also been reported to show a concentration dependence. PL time-resolved measurements at increasing NC concentration have demonstrated a decrease of the decay time,40 whereas FRET results are more efficient for films obtained from higher concentration solution. The analysis of the results for the BSD NC samples is less straightforward. Unfortunately, because the sample is prepared in a single synthetic step, solutions containing pure donors (small NCs) and acceptors (large NCs) and their thin films cannot be prepared for comparison purposes, while their characterization could have helped the interpretation of the observed evidence. However the obtained experimental results suggest that occurrence of FRET in the investigated systems

Figure 8. (a) Time-resolved PL spectra at 1180 nm, of two filmL, obtained from MSD PbS NC sample, starting from toluene (blue) and TCE (red) solutions. (b) Time-resolved PL spectra at 1480 nm, of two filmL, obtained from BSD PbS NC sample, starting from toluene (blue) and TCE (red) solutions. Symbols refer to the excited state decay, while the solid line to the corresponding fitting.

different solvents: toluene and TCE, respectively. The obtained excited state lifetimes, recorded at 1180 nm, have been found of 1.14 ± 0.02 and 1.09 ± 0.02 μs, respectively. Analogously, in Figure 8b is reported the comparison of the lifetime decays of the filmL for the BSD PbS NCs, obtained from the same two solvents. In this case, excited state lifetimes of 1.69 ± 0.22 and 1.84 ± 0.40 μs, respectively, have been recorded at 1480 nm. Interestingly, for both the investigated systems, namely MSD and BSD PbS NCs, the filmL PL decays are comparable for samples obtained from TCE and toluene solutions, irrespectively of the used solvent. In a previous work,28 we found that solvent can influence the final NC arrangement, as the solvent boiling point is related to the solvent evaporation time, and thus also to the time available to the NCs to find their most thermodynamically stable 6150


The Journal of Physical Chemistry C geometrical configuration. However also the solvent polarity plays a role on the interparticle interactions, since attractive interparticle interactions can favor organized geometry, based on short interparticle distances. Therefore, as toluene and TCE have comparable boiling point and polarity, it is reasonable to think that the solid samples fabricated, by using the same building blocks, from TCE and toluene solutions under identical experimental conditions, result in the same arrangement, and consequently similar, as also confirmed by TEM measurements (data not reported).

CONCLUSIONS In this work the spectroscopic characterization of PbS NCs, with both monomodal (MSD) and bimodal size distribution (BSD), has been presented for samples in solution and dropcast on silicon substrates. PbS NCs have shown a sizedependent absorption and PL emission in the NIR range. The well-defined and narrow peaks confirm the PbS NC narrow size-distribution, as indicated by the structural and morphological investigation carried out by TEM and XRD. In addition, time-resolved PL measurements show very long excited state decays, in the microsecond range, which have been attributed to a strong dielectric screening contribution. The NIR PL emission properties of MSD and BSD PbS NCs organized in superlattices have demonstrated to be related to the NC lattice geometry. The careful understanding of the NC positioning in the organized structures, as well as the interparticle distances in the unit crystallographic cell, has allowed a comprehensive interpretation of the steady-state and time-resolved results on thin films of PbS NCs, supporting the occurrence of a FRET between the close-packed NCs only in the case of a MSD distribution. Indeed, although the NC sizedistribution in these sample is narrow, the presence of a small difference in NC size, turns in a concomitant spreading of the energy levels. Therefore the proximity of the NCs in the solid film, together with an effective spectral overlap, make possible the energy transfer. Conversely, in the case of the BSD NC sample, a AB6 geometry is obtained: in this configuration, the energy transfer between neighboring NCs is unlikely to occur for both the large distance between NCs belonging to the same population, in case of larger NCs, and the low overlap between energy level of the two size families. Tuning of the characteristics of BSD PbS NCs, by carefully controlling the synthetic procedures, highly efficient and engineered energy flows in suitably designed NC superlattices can be thus thought to be obtained, resulting in structures with a great potential for light-harvesting, to be employed for example in photovoltaic devices or artificial photosynthesis.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

Scheme of the nanoparticles arrangement in the unit cell in the BSD superlattice (Figure S1) and time-resolved PL spectra of MSD and BSD PbS NC (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

This work has been partially supported by the EU FP7 project “METACHEM” (Grant Agreement CP-FP 228762-2), by the National Laboratory Sens&Micro LAB Project (POFESR 20072013, No. 15) and by the SEED project “X-ray synchrotron class rotating anode microsource for the structural micro imaging of nanomaterials and engineered biotissues (XMILAB)”- IIT Protocol No. 21537 of 23/12/2009.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +39-080-5442128. Tel: +39-080-5442027. Notes

The authors declare no competing financial interest. 6151



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Near Infrared Emission from Monomodal and Bimodal PbS

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